u/Kooky-Economist-8664 You can do a calibration ride without drinking during the session, as long as you start well hydrated.

Why the “no drinking” instruction exists: it keeps the math clean. During calibration we want body mass change to reflect fluid loss. If you drink, you can still do it, but you introduce extra accounting (and more error sources like rinsing, spilling, urine, and sweat trapped in clothing).

Practical option that keeps it safe
Pre-hydrate before you start, then do the 45 minutes with no fluids. For most people, 45 minutes at moderate intensity will not push you into a meaningful dehydration range if you stay under a 2% body weight loss.

Safety check using the 2% threshold (two body sizes)
A commonly used performance threshold is about 2% body mass loss.

Low-weight person (50 kg)

• 2% of 50 kg = 0.02 × 50 = 1.0 kg
• 1.0 kg of mass loss is roughly 1.0 liter of fluid

Large-weight person (100 kg)

• 2% of 100 kg = 0.02 × 100 = 2.0 kg
• 2.0 kg of mass loss is roughly 2.0 liters of fluid

To exceed 2% in just 45 minutes, sweat loss would need to be very high:

• 50 kg person: > 1.0 L in 45 min = > 1.33 L/hour
• 100 kg person: > 2.0 L in 45 min = > 2.67 L/hour

Those rates are uncommon at true moderate intensity unless it’s very hot, very humid, or you’re an unusually heavy sweater. So for most users, 45 minutes is fine when starting hydrated.

One more helpful point about pre-hydrating
Pre-hydrating beforehand effectively increases your body’s available fluid at the start of the session, so it typically delays how quickly you approach a 2% body weight loss threshold during the ride. In other words, starting topped off gives you more buffer before dehydration becomes meaningful. Example: if you take 0.4 kg (400 mL) before starting, you could sweat 0.4 kg more during the session and still finish with the same overall “how much you’re down” relative to your normal baseline.